Concentrated ammonium nitrate solutions are common in chemical processing. For example, ammonium nitrate for use as agricultural fertilizer is produced in significant quantities. Also, metal nitrate solutions such as uranyl nitrate are often precipitated with aqueous ammonia to yield an ammonium nitrate waste stream.
Under certain conditions, molten or crystalline ammonium nitrate may explode due to sudden decomposition. Sudden decomposition of ammonium nitrate releases a large volume of gas and a great quantity of heat, occasionally resulting in a detonating pressure. Historically, many industrial injuries and deaths have been caused by the accidental explosion of ammonium nitrate.
Hazardous explosion of ammonium nitrate is not possible unless three conditions are simultaneously met: (1) concentrated ammonium nitrate; (2) heat; and (3) confinement. Ammonium nitrate susceptible to explosion may be in either liquid or solid form, and total confinement is not necessary. While explosive decomposition occurs at 260.degree. C. for pure ammonium nitrate, certain chemical sensitizers lower the decomposition temperature. Also, chemical process design features that avoid concentration, heat, and confinement help ensure safety despite the possibility of human error. These features include temperature and flow interlocks to ensure unblocked pump flow, and piping details that prevent inadvertent blockage.
Ammonium nitrate decomposes according to either of two reactions under different conditions. Under low-pressure conditions such as atmospheric, NH.sub.4 NO.sub.3 decomposes rather harmlessly into ammonia and nitric acid: EQU NH.sub.4 NO.sub.3 .fwdarw.NH.sub.3 +HNO.sub.3
Under confining conditions which allow buildup of high pressure, the decomposition occurs according to a hazardous reaction called rapid thermal decomposition: EQU NH.sub.4 NO.sub.3 (aqueous).fwdarw.N.sub.2 O (gas)+2H.sub.2 O (gas)
The latter reaction occurs extremely rapidly, creating a pressure wave which accounts for the explosion. This reaction also produces significant heat, which further increases the decomposition rate and also amplifies the gas pressure. Once the reaction is initiated, decomposition may accelerate such that the time scale for the entire event is just several milliseconds. The confining conditions which allow pressure to build up from the hazardous decomposition do not need to be total. In fact, a partially blocked pump discharge is sufficient to create the hazard of explosion.
Centrifugal pumps, such as those conventionally used to pump ammonium nitrate during chemical processing, are capable of creating all of the conditions necessary for hazardous ammonium nitrate explosions. A partially blocked pump can create heat to gradually increase the temperature until the reaction is initiated. This heat also allows the second of the three conditions to occur, i.e., concentration of the ammonium nitrate. Finally, partial or total pump blockage may result in sufficient confinement to allow rapid pressure build up and the potential for explosion.
Depending on the acceleration rate of the rapid thermal decomposition, the explosion may be of two types. In the first, most common, type of explosion, the pressure wave simply ruptures process equipment in proximity. However, an extremely high rate of decomposition may result in a pressure wave which moves at the speed of sound (e.g. sonic); such a pressure wave is completely unyielding and extremely powerful. This class of explosion is a detonation that can disintegrate process equipment into tiny pieces with high force.
Pure ammonium nitrate decomposes by hazardous thermal decomposition at 260.degree. C. and 1422 psi. Impurities may significantly affect the decomposition temperature, rate, or potential. For example, the presence of certain sensitizing materials, such as metal particles (especially aluminum), wax, chlorinated materials, organics including hydrocarbons and wood, and other compounds, lowers the temperature and pressure thresholds required for an explosion. Certain substances such as calcium oxide (lime) stabilize ammonium nitrate and increase the temperature and pressure thresholds.
Safety standards recommend designs that limit the temperature of ammonium nitrate solutions, using electronic instrumentation and other features to prevent blocked flow. One such standard recommends the use of instrumentation to control temperatures of highly concentrated solutions below 370.degree. F. (188.degree. C.). The recommended interlock stops the pump when the liquid temperature nears the decomposition temperature of ammonium nitrate.
Conventional instrumentation loops to provide this control are expensive to install and maintain, and must be periodically calibrated and functionally tested. An analog temperature interlock typically consists of a thermocouple or resistive temperature device located in piping near the pump. The thermocouple is connected to an electronic transmitter, which is in turn connected via coaxial cable to a control system. The control system may be either a distributed control system or a current switch which controls the motor start circuit. Each component in this conventional system is complex and requires periodic maintenance and calibration.
Somewhat simpler temperature switches are used in piping adjacent to pumps. These switches usually include a bimetallic element which opens an electrical circuit and stops the pump. However, these switches must be periodically calibrated and tested. Also such temperature switches are not mounted directly on the pump casing, which is where high pump temperatures can be most reliably detected.